The enterococcal bacteria, including Enterococcus faecalis, are commensal Gram-positive bacteria colonizing the intestines of human and other animal hosts. These bacteria have been recognized as a common cause of endocarditis since the early 1900s, and in the past two decades as opportunistic pathogens that can lead to serious nosocomial infections [1]. They are also associated with many clinical infections in humans including septicemia, bacteremia, and various urinary tract infections. E. faecalis has many intrinsic and acquired antibiotic resistances that have long been known to complicate therapy of endocarditis, and during recent years resistances to almost all commercially available antibiotics have appeared, making the development of new therapies against the enterococci all the more important.
Without new therapies to treat or prevent enterococcal infections, health care providers may be left without any effective means to treat serious infections caused by the emerging multi-drug resistant enterococci. New and alternative strategies to treat or prevent these infections are thus clearly needed.
Adherence of pathogenic bacteria to the host tissue, mediated by adhesins, is the first event in a multi-step process that may lead to clinically manifested infections. For organisms such as Staphylococcus aureus and E. faecalis, which are primarily extracellular pathogens, ECM (extracellular matrix—see footnote 1) components are the targets for adherence. MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) represent a subfamily of bacterial adhesins that recognize and bind to ECM components. Several MSCRAMMs have been isolated and characterized from staphylococci and streptococci [2,3], among them the S. aureus collagen-binding MSCRAMM, Cna, such as disclosed in U.S. Pat. No. 6,288,214, incorporated herein by reference.
Cna is a mosaic protein with a molecular mass of 135 kDa (FIG. 1c) [4-8]. This protein features an N-terminal signal sequence followed by a 500-residue long A domain of unique amino acids sequence and a B domain that contains a 110-residue long unit repeated tandemly one to four times in Cna isolated from different strains of S. aureus [9]. The C-terminal region of Cna contains a cell wall-associated domain, which includes the LPXTG motif that is a putative recognition site for the hypothetical enzyme sortase that covalently links Cna to the cell wall [4]. A hydrophobic transmembrane region is followed by a short cytoplasmic tail rich in positively charged residues. Earlier work showed that the presence of Cna is necessary and sufficient to allow S. aureus cells to adhere to collagenous tissues such as cartilage [10], and Cna was shown to be a virulence factor in experimental septic arthritis [11]. Vaccination of mice with a recombinant form of the Cna A domain protected against induced staph sepsis [6].
However, present knowledge of the molecular pathogenesis of enterococcal infections is very limited, and there is a distinct need in the field to develop compositions and methods to address the serious problems presented by enterococcal infection. In addition, despite the fact that it has been shown that clinical isolates can adhere to ECM proteins such as collagen, laminin, and fibrinogen [12,13], the MSCRAMMs involved have not been previously identified, and thus there have not been any collagen-binding proteins isolated which can prevent or treat infections caused by enterococcal bacteria.